This Week on Galileo
August 27 - September 2, 2001
Galileo Concentrates on Cruise Activities
This is a relatively busy week for Galileo, considering that we are in the
cruise portion of our 31st orbit of Jupiter. On Monday, the Near Infrared
Mapping Spectrometer (NIMS) performs a calibration. The instrument, which
can measure the temperatures of the features it looks at, will direct its
gaze towards a target plate mounted on the spacecraft that has been heated
to a known temperature. Measuring the signal the instrument generates will
allow scientists to accurately gauge the true temperatures of the
atmospheric and surface scenes that were captured by NIMS during the August
5 flyby of Jupiter and Io.
On Thursday, routine maintenance of the on-board tape recorder is
performed. At least once every 30 days, the tape is wound at high speed
from one end to the other and back again to help reduce the mechanical
stickiness which has plagued the operation of the recorder in the past.
Normal playback operations consist of many small low-speed motions back and
forth, as small amounts of data are read into the spacecraft computer
memory, processed, and packaged for transmission to Earth at rates of 20 to
160 bits per second. When you compare this to a typical computer modem,
which can communicate at 56,000 bits per second, you can begin to see why
it takes a month or two for Galileo to completely read out a full tape,
which can contain nearly a gigabit (1,000,000,000 bits) of data. At the
modem speed, this amount of data could be transmitted in approximately 5 hours.
On Saturday, Galileo reaches a milestone as it performs the 100th scheduled
orbit trim maneuver since entering orbit around Jupiter in December of
1995. This engine burn will fine-tune the trajectory of the spacecraft,
directing its path to the next flyby of Io in October. The maneuver, which
could last as long as 9 hours, is preceded by an automatic drift rate
calibration of the gyroscopes, which are used to maintain the attitude of
the spacecraft as the thrusters fire.
On Sunday, routine maintenance of the propulsion system is performed,
followed by a 4 degree turn of the spacecraft to keep the communications
antenna pointed towards Earth.
Io flyby data scheduled for tape playback this week are two NIMS
observations and one from the Photopolarimeter Radiometer instrument (PPR).
The NIMS observations are of the Gish Bar and Amirani hot spots on Io,
measuring temperatures and sulfur dioxide distribution in those areas. The
PPR measurement is a temperature map of Io along a single strip that runs
from the north to the south pole, along a line representing roughly noon
relative to the 42-hour "day" of the satellite. These observations were
taken an hour to an hour and a half after the closest approach to Io on